Nonemissive displays convey information using contrast differences, which are achieved by varying the reflectance or transmission of light; they are thus distinct from traditional emissive displays, which stimulate the eye by emitting light. One type of nonemissive display is an electrophoretic display, which utilizes the phenomenon of electrophoresis to achieve contrast. Electrophoresis refers to movement of charged particles in an applied electric field. When electrophoresis occurs in a liquid, the particles move with a velocity determined primarily by the viscous drag experienced by the particles, their charge (either permanent or induced), and the magnitude of the applied field.
An electrophoretic display utilizes charged particles of one color suspended in a dielectric liquid medium of a different color (that is, light reflected by the particles) is absorbed by the liquid. The suspension is housed in a cell located between (or partly defined by) a pair of oppositely disposed electrodes, one of which is transparent. When the electrodes are operated to apply a DC or pulsed field across the medium, the particles migrate toward the electrode of opposite sign. The result is a visually observable color change. In particular, when a sufficient number of the particles reach the transparent electrode, their color dominates the display; if the particles are drawn to the other electrode, however, they are obscured by the color of the liquid medium, which dominates instead.
Ideally, the particles maintain a strong uniform charge throughout the lifetime of the device and move as rapidly as possible under the influence of a relatively small electric field. The "switching time" t of suspended particles located between two electrodes, i.e., the time required for the population of particles to migrate from one of the electrodes to the other, is given by ##EQU1##
where d is the spacing between electrodes, .eta. is the viscosity of the liquid medium, .di-elect cons. is its dielectric constant, V is the potential difference between the electrodes, and .zeta. is the zeta potential of the particles. Thus, the system is usually selected to minimize t. For example, the spacing between electrodes is only as large as is necessary to ensure that the particles are completely obscured following migration away from the transparent electrode.
Useful electrophoretic displays are bistable: their state persists even after the activating electric field is removed. This is generally achieved via residual charge on the electrodes and van der Waals interactions between the particles and the walls of the electrophoretic cell. As disclosed in U.S. Ser. Nos. 08/738,260, 08/819,320 and 08/935,800, and PCT application Ser. No. US96/13469, the entire disclosures of which are hereby incorporated by reference, electrophoretic displays may be fabricated from discrete, microencapsulated electrophoretic 10 elements. This approach eliminates the effects of agglomeration on a scale larger than the size of the capsule, which preferably is sufficiently small to be individually unnoticeable. Thus, the capsules function in a manner similar to pixels (although typically they are not individually addressable); even if agglomeration occurs, its effect is confined to a very small area. Furthermore, by setting an upper limit to the possible size of an agglomeration--that is, by preventing accumulations larger than the particle content of a capsule--the bulk effects of diminished field responsiveness and vulnerability to gravity are likewise limited.
Electrophoretic displays in accordance with the '260 application are based on microcapsunes each having therein an electrophoretic composition of a dielectric fluid and a suspension of particles that visually contrast with the dielectric liquid and also exhibit surface charges. A pair of electrodes, at least one of which is visually transparent, covers opposite sides of a two-dimensional arrangement of such microcapsules. A potential difference between the two electrodes causes the particles to migrate toward one of the electrodes, thereby altering what is seen through the transparent electrode. When attracted to this electrode, the particles are visible and their color predominates; when they are attracted to the opposite electrode, however, the particles are obscured by the dielectric liquid.
This approach is well-suited to applications involving contiguous arrays of electrophoretic elements intended to change state in unison. More difficult are applications requiring imposition of a visible pattern by selective activation of elements in the array. Imaging, in this sense, requires the ability to selectively apply electric fields of small spatial extent and high magnitude. The dimensions of the field effectively determine the resolution of the applied pattern, while the field magnitude dictates the switching time of the display and, therefore, the speed at which imaging can occur. Of course, the imaging speed is also limited by the rate at which the field itself can be toggled between high and low states.
Printer-type applications capable of imaging, at realistic rates, substrates bearing a multitude of small electrophoretic display elements may require fields on he order of 1 V/.mu.m. Generating such fields rapidly, and controlling them with conventional digital logic devices that operate at low voltages, represents a significant design challenge.